Neutrinos in the early universe

The contribution of neutrinos to the energy density of the universe is negligible. Instead, neutrinos influence significantly non-equilibrium processes in the early universe: the formation of structure, nucleosynthesis and baryogenesis. From the cosmic microwave background and from the large scale structure of matter one infers an upper bound of 1 eV on the sum of neutrino masses. The analysis of the abundances of light elements determines the number of neutrino flavours to be N ν = 3 . Finally, decays of heavy Majorana neutrinos at very high temperatures can naturally explain the cosmological baryon asymmetry. Successful baryogenesis, independent of initial conditions, is possible for neutrino masses in the range 10 −3 ⩽ m i ≲ 0.1 . This mass window is consistent with results from neutrino oscillations and will soon be probed by cosmological observations. To cite this article: W. Buchmüller, C. R. Physique 6 (2005).